Vacuum UV Range Research Articles

Flux and fluence effects on the vacuum-UV photodesorption and photoprocessing of CO2 ices.

CO2 is a major component of the icy mantles surrounding dust grains in planet and star formation regions. Understanding its photodesorption is crucial for explaining gas phase abundances in the coldest environments of the interstellar medium irradiated by vacuum-UV (VUV) photons. Photodesorption yields determined experimentally from CO2 samples grown at low temperatures (T = 15 K) have been found to be very sensitive to experimental methods and conditions. Several mechanisms have been suggested for explaining the desorption of CO2, O2 and CO from CO2 ices. In the present study, the cross-sections characterizing the dynamics of photodesorption as a function of photon fluence (determined from released molecules in the gas phase) and of ice composition modification (determined in situ in the solid phase) are compared for the first time for different photon flux conditions (from 7.3 × 1012 photon per s cm-2 to 2.2× 1014 photon per s cm-2) using monochromatic synchrotron radiation in the VUV range (on the DESIRS beamline at SOLEIL). This approach reveals that CO and O2 desorptions are decorrelated from that of CO2. CO and O2 photodesorption yields depend on photon flux conditions and can be linked to surface chemistry. In contrast, the photodesorption yield of CO2 is independent of the photon flux conditions and can be linked to bulk ice chemical modification, consistently with indirect desorption induced by an electronic transition (DIET) process.

Vacuum-UV Photodesorption from Compact Amorphous Solid Water: Photon Energy Dependence, Isotopic and Temperature Effects

Photodesorption from water-rich interstellar ice mantles is known to play an important role on the gas-to-ice ratio in stars and planet formation regions. Quantitative determination of the photodesorption yields in the laboratory is crucial to astrochemical models. This study presents for the first time, the photon-energy dependence of the photodesorption yields from water ice samples in the vacuum-UV (VUV) range. Experiments have been performed with the Surface Processes and ICES (SPICES) setup coupled to the DESIRS beamline at the SOLEIL synchrotron facility (St. Aubin, France). Thick (20–100 ML) compact amorphous solid water ices (H2O and D2O) grown onto a cold Au substrate have been irradiated at sample temperatures ranging from 15 to 110 K. Photodesorption yields of water and photoproducts have been obtained by mass-spectrometry from 7 to 13.5 eV. In interstellar conditions, average H2O photodesorption yields are (5 ± 2) × 10–4 molecule/photon at low temperature (15 K) whereas, lower yields, by a factor of ∼6–9 on average, were found for D2O. This strong isotopic effect can be explained by a differential chemical recombination between OH (OD) and H (D) photofragments at the surface of the samples. In addition, an enhancement of the yields above (70 ± 10) K suggests a thermally induced ice restructuration at this threshold temperature.

Self-consistent dielectric functions of materials: Toward accurate computation of Casimir-van der Waals forces.

Research on theoretical calculation of Casimir-van der Waals (vdW) forces is characterized by a great number of inconsistencies and conflicting reports with widely differing results for many known materials, including water, contradicting experimental measurements. Despite its importance for conceptual advances in both fundamental aspects and practical applications, a universal framework for the accurate determination of Casimir-vdW forces is lacking. Here, we propose a universal theoretical platform for computing Casimir-vdW forces, accounting for the electronic dielectric constant, optical bandgap, density, and chemical composition. Using this methodology, we determine the dielectric function for 55 materials, over a wide range of photon energies, covering an extensive list of common metals, organic and inorganic semiconductors, and insulators. Internal consistency of the compiled data is validated using optical sum rules and Kramers-Kronig relations. We demonstrate that the calculated vdW forces based on these data match remarkably well with the experimentally measured vdW forces.

Detecting sulfur on the Moon: The potential of vacuum ultraviolet laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) receives increasing interest in the field of space exploration. Some volatile elements important to planetary research are, however, challenging to detect with conventional LIBS because their strongest emission lines lie in the vacuum-UV (VUV) spectral range below 200nm. We demonstrate the potential of LIBS detection in the VUV range from 125 to 190nm on one of the relevant challenging elements, sulfur (S), in a lunar context. With our laboratory set-up, we detected S in lunar analogues at concentrations ranging from 0.5 to 4.0at% in a time-integrated configuration. The experiments were conducted in high vacuum (10−3 Pa) with laser parameters realistic for a space instrument (1064nm, 25mJ, 6ns). The results indicate improved detection capabilities for S in a lunar geological context as compared to LIBS in the more common UV/VIS/NIR spectral range. We found limits of detection in the order of 0.5at% S for all investigated sample series. Normalization by total intensity and by an internal standard, in our case by the emission signal of the O I triplet at 130nm, was tested as well, but did not yield a noticeable improvement for our data. Besides S, we were also able to detect Al, Fe, O, Si and Ti from the lunar analogue matrix.

The vacuum UV photoabsorption spectroscopy of the cis-1,2-dichloroethylene (1,2-ClHC=CHCl) in the 5-20 eV range. An experimental and theoretical investigation

The photoabsorption spectrum of cis-1,2-C2H2Cl2 has been examined in detail in the vacuum UV range between 5 eV and 20 eV photon energy by using synchrotron radiation. Quantum chemical calculations are proposed and applied to the electronic transitions and to the vibrational structures belonging to these transitions. The broad band observed at 6.568 eV includes the X̃1A1→11B1, 31B2 and 21B1 transitions. The two latter excitations correspond to the valence 2b1(π)→π* and to the Rydberg 2b1(π)→3s transitions. The former excitation is described by a more complex 2b1(π)→nCl +σ*CH/Rpσ transition where valence and Rydberg characters are strongly mixed. For these transitions short vibrational progressions are observed, analyzed and tentatively assigned. The abundant structure observed between 7.0 eV and 10.0 eV has been analyzed in terms of vibronic transitions to one ns- (δ¯ = 0.960), two np- (δ¯ = 0.525 and 0.337), and two nd-type (δ¯ = 0.080 and 0.002) Rydberg series, all converging to the X̃2B1 ionic ground state. The vibrational structure analysis of the Rydberg states leads to the following average wave numbers: ω2 ≈ 1420 cm-1 (C=C stretching), ω3 ≈ 1190 cm-1 (symmetric C-H bending), ω4 ≈ 800 cm-1 (symmetric C-Cl stretching) and ω5 ≈ 190 cm-1 (symmetric C-Cl bending). These numbers are compared to previously reported values. Many other transitions are observed between 10 eV and 20 eV and are assigned to transitions to Rydberg states converging to the successive excited states of cis-1,2-C2H2Cl2+. For several of these Rydberg states, a vibrational structure is also observed and interpreted.

Polarization Calibration of the Chromospheric Lyman-Alpha SpectroPolarimeter for a 0.1% Polarization Sensitivity in the VUV Range. Part II: In-Flight Calibration

The Chromospheric Lyman-Alpha SpectroPolarimeter (CLASP) is a sounding rocket experiment designed to measure for the first time the linear polarization of the hydrogen Lyman- ${\alpha}$ line (121.6 nm) and requires a $0.1~\%$ polarization sensitivity, which is unprecedented for a spectropolarimeter in the vacuum UV (VUV) spectral range. A unique polarization calibration experiment was conducted under vacuum conditions to estimate the response matrix of the instrument. For this purpose, a custom-made light source was designed to inject Lyman- ${\alpha}$ light with a known linear polarization state into the spectropolarimeter. Two methods were employed to change the orientation of the linear polarization input: one by rotating the light-source itself (direct method), the other by rotating a half-waveplate located after the light-source’s polarizers (waveplate method). The spurious polarization, scale factor, and azimuth error terms of the response matrix were successfully estimated from the polarization calibration measurements. However, it was found that the direct method could not provide an accuracy better than $0.1~\%$ on the spurious polarization terms, whereas their required tolerance was ${<}\,0.017~\%$ . On the other hand, the waveplate method determined these terms with only a ${\sim}\,0.04~\%$ accuracy due to residual cross-talk between polarization and intensity. Nevertheless, the polarization calibration confirmed the very low spurious polarization level of the instrument, which will also be confirmed with the flight data. The resulting response matrix deviated from an ideal one, and possible causes of the deviation are discussed by considering the polarization properties of the optical components.

Anisotropy-Guided Enantiomeric Enhancement in Alanine Using Far-UV Circularly Polarized Light.

All life on Earth is characterized by its asymmetry - both the genetic material and proteins are composed of homochiral monomers. Understanding how this molecular asymmetry initially arose is a key question related to the origins of life. Cometary ice simulations, L-enantiomeric enriched amino acids in meteorites and the detection of circularly polarized electromagnetic radiation in star-forming regions point to a possible interstellar/protostellar generation of stereochemical asymmetry. Based upon our recently recorded anisotropy spectra g(λ) of amino acids in the vacuum-UV range, we subjected amorphous films of racemic (13)C-alanine to far-UV circularly polarized synchrotron radiation to probe the asymmetric photon-molecule interaction under interstellar conditions. Optical purities of up to 4% were reached, which correlate with our theoretical predictions. Importantly, we show that chiral symmetry breaking using circularly polarized light is dependent on both the helicity and the wavelength of incident light. In order to predict such stereocontrol, time-dependent density functional theory was used to calculate anisotropy spectra. The calculated anisotropy spectra show good agreement with the experimental ones. The European Space Agency's Rosetta mission, which successfully landed Philae on comet 67P/Churyumov-Gerasimenko on 12 November 2014, will investigate the configuration of chiral compounds and thereby obtain data that are to be interpreted in the context of the results presented here.

Powder Reflection Spectroscopy in the Vacuum uv range

The measurement of diffuse refl ectance spectra below 230 nm for ceramic luminescent materials is usually not performed although it is of tremendous interest, e.g., to determine their quantum effi ciency upon VUV excitation. In contrast, the recording of VUV excitation spectra is well established, e.g., to evaluate VUV phosphors. The lack of access to diffuse powder refl ection spectra in this range is caused by the absorption edge (optical band gap) of the most common refl ectance standards; for instance, BaSO4 or CaCO3–VUV photons with energies higher than the absorption edge are not refl ected but absorbed and thus not amenable to detection in a synchronous scan. The measurement technique presented in this paper allows monitoring refl ection properties of powder samples in the range from 120 to 300 nm (10.3 to 4.1 eV). To this end, all VUV photons refl ected by the sample are converted into visible photons, which are collected in an Ulbricht sphere and fi nally counted by a photomultiplier tube (PMT). This requires a custom-built integrating sphere, viz. one that is coated with a (V)UV to Vis converter, e.g., a phosphor such as BaMgAl10O17:Eu2+ (BAM:Eu) showing a spectrally independent and high light output in the respective wavelength range. This paper sketches the technique and procedure for recording VUV refl ection spectra and shows the results for several standard phosphors. It also reveals the limits of this new technique.

Reflectance difference spectroscopy of water on Cu(110)

The adsorption of H2O on Cu(110) was probed by means of reflectance difference spectroscopy (RDS) in the energy range between 1.5 and 9.3eV and by scanning tunneling microscopy (STM). The adsorption of water on the pristine Cu(110) substrate mainly induces changes in the Cu surface state related optical transitions. Furthermore, the H2O adsorbate modifies the Cu bulk transitions. In particular, our investigations demonstrate that the coverage-dependent phase transition from 1D pentamer chains to a (7×8) superstructure can be monitored by means of RDS. In the vacuum-UV range, new RD features assigned to Cu bulk transitions were detected. Adsorption on the metal surface strongly modifies or quenches the H2O HOMO–LUMO transition, whereby a distinct RD feature of the water molecules themselves in the vacuum-UV range is absent.

ChemInform Abstract: The Quantum Cutting of Tb3+ in Ca6Ln2Na2 (PO4)6F2 (Ln: Gd, La) under VUV—UV Excitation: With and Without Gd3+.

AbstractThe phosphors Ca6Gd2‐xTbxNa2 (PO4)6F2 show strong absorption in the vacuum UV (VUV) range and have intense emission upon 172 nm excitation, while Ca6La2‐xTbxNa2 (PO4)6F2 phosphors exhibit weaker absorption in the VUV range.

Laser-induced optical activity in range of Rydberg autoionizing states of xenon

Optical activity of xenon atoms in the vacuum UV range induced by circularly polarized laser light is studied theoretically. The optical activity arises in the vicinity of the autoionizing state 5p5(2P1/2)8s′\( \left[ {\frac{1} {2}} \right]_1 \) as a result of its coupling via the laser field with the discrete state 5p5(2P3/2)7p\( \left[ {\frac{1} {2}} \right]_1 \). Polarization variations of the vacuum UV radiation upon its propagation through the atomic medium are calculated, and the possibility of controlling this polarization is discussed. Manifestations of nonresonant coupling of the discrete state with the broad autoionizing state 5p5(2P1/2)6d′\( \left[ {\frac{1} {2}} \right]_1 \) induced by the overlap of the Rydberg autoionizing series in xenon are studied.

WSO/UV spectrographs: the expected performance of HIRDES

The World Space Observatory Ultraviolet (WSO/UV) is a multi-national project grown out of the needs of the astronomical community to have future access to the UV range. WSO/UV consists of a single UV telescope with a primary mirror of 1.7 m diameter feeding the UV spectrometer and UV imagers. The spectrometer comprises three different spectrographs, two high-resolution echelle spectrographs (the High-Resolution Double-Echelle Spectrograph, HIRDES) and a low-dispersion long-slit instrument. Within HIRDES the 102–310 nm spectral band is split to feed two echelle spectrographs covering the UV range 174–310 nm and the vacuum-UV range 102–176 nm with high spectral resolution (R>55000). The technical concept is based on the heritage of two previous ORFEUS SPAS missions. The phase-B1 development activities are described in this paper considering the performance of the instruments.

Low-temperature low-damage sterilization based on UV radiation through plasma immersion

This paper introduces a new type of high-frequency (HF) sustained discharge where the HF field applicator is a planar transmission line that allows us to fill with plasma a long chamber of rectangular cross-section (typically 1 m × 15 cm × 5 cm). Peculiar interesting features of this plasma source are a low gas temperature (typically below 40 °C in the 1 Torr range in argon), broadband impedance matching with no need for retuning, stability and reproducibility of the discharge (non-resonant behaviour). This type of plasma source could be useful for web processing; nonetheless, it is applied here to plasma sterilization, taking advantage of its low gas temperature to inactivate microorganisms on polymer-made medical devices to avoid damaging them. The predominant biocide species are the UV photons emitted by the discharge whereas most plasma sterilization techniques call for reactive species such as O atoms and OH molecules, which induce significant erosion damage on polymers. Polystyrene microspheres are actually observed to be erosion-free under the current plasma sterilization conditions (scanning electron micrographs have been examined). Moreover, inactivation is quite fast: 106 B. atrophaeus spores deposited on a Petri dish are inactivated in less than 1 min. Correlation of the UV radiation with the spore inactivation rate is examined by (i) considering the emitted light intensity integrated over the 112–180 nm vacuum UV (VUV) range with a photomultiplier; (ii) looking with an optical spectrometer at the emission spectrum over the 200–400 nm UV range; (iii) using absorption spectroscopy to determine the role of the VUV argon resonant lines (105 and 107 nm) on spore inactivation. It is found that the test-reference spores are mainly inactivated by VUV photons (112–180 nm) that are primarily emitted by impurities present in the argon plasma.

HIRDES – The High-Resolution Double-Echelle Spectrograph for the World Space Observatory Ultraviolet (WSO/UV)

The World Space Observatory Ultraviolet (WSO/UV) is a multi-national project grown out of the needs of the astronomical community to have future access to the UV range. WSO/UV consists of a single UV telescope with a primary mirror of 1.7 m diameter feeding the UV spectrometer and UV imagers. The spectrometer comprises three different spectrographs, two high-resolution echelle spectrographs (the High-Resolution Double-Echelle Spectrograph, HIRDES) and a low-dispersion long-slit instrument. Within HIRDES the 102–310 nm spectral band is split to feed two echelle spectrographs covering the UV range 174–310 nm and the vacuum-UV range 102–176 nm with high spectral resolution ( R > 50,000). The technical concept is based on the heritage of two previous ORFEUS SPAS missions. The phase-B1 development activities are described in this paper considering performance aspects, design drivers, related trade-offs (mechanical concepts, material selection etc.) and a critical functional and environmental test verification approach. The current state of other WSO/UV scientific instruments (imagers) is also described.

Main Results of the SPIRIT Experiment Onboard the CORONAS-F Satellite

The main results of the SPIRIT experiment on imaging spectroscopy of the Sun in the soft X-ray and extreme vacuum UV range are presented. The results were obtained onboard the CORONAS-F satellite, which has been operating since July 2001. More than 40 thousand observation sessions were performed during the experiment. About a million solar images and spectra (more than 250 Gb of information) were obtained, including monotemperature images of the solar atmosphere in six spectral regions, corresponding to temperatures from 0.05 to 2 MK; full-Sun spectral images (spectroheliograms) in more than 150 lines (177–207 A and 285–335 A, T from ∼0.05 to 20 MK); images of the full Sun in the monochrome Mg XII line (8.42 A, T ∼ 10 MK); images of the solar corona at a distance of up to five solar radii; continuous series (up to 20 days long) of observations with high time resolution (40–100 s); observations of the flare dynamics, including the preflare, initial, and main phases, with a resolution of 7 s, and data on the absorption of X-ray and XUV solar radiation in the upper atmosphere of the Earth. The study was performed for the maximum of the 11-year solar activity cycle and for its decrease phase.

Relaxation behavior of rovibrationally excited H2 in a rarefied expansion

The evolution of the rotational and vibrational distributions of molecular hydrogen in a hydrogen plasma expansion is measured using laser induced fluorescence in the vacuum-UV range. The evolution of the distributions along the expansion axis shows the relaxation of the molecular hydrogen from the high temperature in the upstream region to the low ambient temperature in the downstream region. During the relaxation, the vibrational distribution, which has been recorded up to v = 6, is almost frozen in the expansion and resembles a Boltzmann distribution at T approximately 2200 K. However, the rotational distributions, which have been recorded up to J = 17 in v = 2 and up to J = 11 in v = 3, cannot be described with a single Boltzmann distribution. In the course of the expansion, the lower rotational levels (J < 5) adapt quickly to the ambient temperature ( approximately 500 K), while the distribution of the higher rotational levels (J > 7) is measured to be frozen in the expansion at a temperature between 2000 and 2500 K. A model based on rotation-translation energy transfer is used to describe the evolution of the rotational distribution of vibrational level v = 2 in the plasma expansion. The behavior of the low rotational levels (J < 5) is described satisfactory. However, the densities of the higher rotational levels decay faster than predicted.

Spectral heterogeneity of oxygen-deficient centers in Ge-doped silica

We report an experimental investigation of the emission spectra of a 1000molppm sol–gel Ge-doped silica by fine tuning the excitation energy in the ultraviolet (UV) range, around 5eV, and in the vacuum-UV range, around 7.3eV, at room temperature and at 10K. The sample is characterized by a blue (centered at ∼3.2eV) and an UV (centered at ∼4.3eV) bands. We have found that the ratio between the area of the blue and the UV bands depends on the temperature and on the excitation energy in both the vacuum-UV and the UV range. At both temperatures the spectral features of the blue and the UV bands are weakly affected when the excitation is varied in the vacuum-UV. At variance, under UV excitation the peaks of the bands are shifted and also their widths are changed. These results are interpreted in terms of distinct excitation channels of the luminescence that are influenced in a different way by the structural inhomogeneity of point defects.

Photoluminescence activity in natural silica excited in the vacuum-UV range

We report an experimental study on the optical absorption and photoluminescence detected in samples of natural silica. Our results show that the two emission bands, β (∼3.1 eV) and α E (∼4.3 eV), have an excitation profile in the vacuum ultraviolet region with a maximum at ∼7.5 eV. This excitation profile indicates that, in terms of energy levels of the luminescent defect, there is a transition from a ground state, S 0, to a second excited state, S 2, able to excite PL emission, in addition to the well known transition corresponding to the optical absorption band, B 2β. Our data are in a quantitative agreement with `ab initio' calculations carried out for a two-fold coordinated Ge defect in SiO 2 and exclude a correlation between the S 0 → S 2 transition and the 7.6 eV E band.

Characterization of a charge-coupled-device detector in the 1100–014-nm (1-eV to 9-keV) spectral region

The performances of a CCD detector have been evaluated in a wide spectral region, which ranges from the near IR to the soft x ray. Four different experimental setups have been used: a Czerny-Turner monochromator for the 1100-250-nm region, a normal-incidence Johnson-Onaka monochromator for the 250-30-nm region, a grazing-incidence Rowland monochromator for the 30-0.27-nm region, and a test facility with broadband filters for the 0.27-0.14-nm region. The CCD is thinned and backilluminated, with a 512 x 512 format and 24 microm x 24 microm pixels. The quantum efficiency was measured in the 1100-0.14-nm (1-eV to 9-keV) region, and the uniformity of response was in the 1100-58-nm (1-21-eV) region. Contamination effects in the vacuum UV range are also discussed.

A continuously-tunable picosecond laser radiation source in the UV spectral region

We present the results of research on continuous tuning of a neodymium laser radiation frequency in the UV and vacuum ranges. We obtained generation of the total frequency of the waves of the second harmonic and of parametric light generator radiation in the UV region (338–366 nm). We investigate the optimum conditions for tuning UV radiation in the 113.5–117.0 nm range in the process of generation of the third harmonic in xenon and its mixtures with other gases. In the vacuum UV range investigated, an efficiency of generation of the third harmonic of ∼5·10−4 and a tuning range above 2600 cm−1 are realized.